For example, in metal secondary working industries such as automobile industries, the compatibility of higher workability with strength is required with respect to a metal plate which is the object of working. Specifically, in the above-mentioned automobile industries, from the need to make the body light in weight in order to seek low fuel consumption in view of the earth environmental problem which has been raised recently, there is a requirement of a steel plate which has a higher strength while maintaining a deep drawing property provided heretofore.
As evaluation indices for such a metal plate, for example, an elongation index, a deep drawing property, an aging index, a strength, a secondary working brittleness, a baking hardening property, a spot welding property, etc., may be considered. Thus, when the deep drawing property is evaluated by a Lankford value (hereinafter referred to as r value: metal plate width strain/plate thickness strain) by placing great importance on the deep drawing property, it is known that the reduction of the amount of carbon (hereinafter referred to as C) in the steel is most advantageous, and in addition, by this low carbonization, the elengation index (EI) and the cold-slow-aging index (AI: the lower the AI, the better) are also improved. However, on the other hand, when the amount of C in the steel decreases, most of the other evaluation indices are deteriorated. For example, since the structure strength is lowered due to reduction of precipitation, a tensile strength (ST) is decreased, and since the grain boundary strength is lowered, the secondary working brittleness is deteriorated, and since the amount of solid solution C is reduced, the baking hardening property is deteriorated. Furthermore, when the amount of C in the steel is equal to or lower than 50 ppm, the grain growth rate is promoted by heating of welding, and due to the grain coarsening in a heat affected zone (HAZ), the spot welding property is deteriorated.
The present applicant developed a continuous annealing and carburizing facility as described in Japanese Patent Laid-Open Publication Hei No. 4-88126 as shown in FIG. 2 in order to improve the above-mentioned tensile strength, secondary working brittleness, BH property, and spot welding property by making the solid solution C exist in a surface layer portion by a continuous carburizing treatment, subsequent to a continuous annealing treatment of a metal strip consisting of extremely low carbon steel as shown in FIG. 1 wherein the above-mentioned elongation index, deep drawing property, and cold-slow-aging index are obtained by recrystallizing and annealing.
In this continuous annealing and carburizing facility, after performing a predetermined recrystallizing and annealing with respect to a metal strip (strip A) in a preheating region 1 and a heating region 2, or a uniformly heating region 3, a carburizing treatment is performed in a carburizing region 4 by controlling a metal strip temperature, atmospheric factors, a transportation speed (in-turnace time) and cooling conditions, so that it is possible to continuously manufacture the metal strip having desired values (form) of a surface carburizing depth and a concentration distribution while satisfying material characteristic specifications of the metal strip.
On the other hand, as the method for controlling the distribution form of the surface carburizing depth and the concentration distribution of the surface layer portion of the metal strip, a method is described in Japanese Patent Publication No. 54-31976. In this control method of the carburizing depth and the concentration distribution, a carburizing gas is jetted and introduced at a predetermined flow rate in a carburizing period in order to infiltrate carbon into the surface layer portion of the metal strip, and in a diffusion period following to the carburizing period, under a sufficiently reduced pressure with the carburizing gas exhausted, the infiltrated carbon is diffused to the surface layer portion of the metal strip. And the carburizing concentration distribution form consisting of the carburizing depth and the carburizing concentration is controlled by controlling time periods of the carburizing period and the diffusion period. In this control method of the carburizing depth and the carburizing concentration, it is possible to prevent non-uniform carburizing which is apt to occur in a gas jet carburizing which requires in particular, a thin carburized layer (carburized case).
However, in setting various conditions of such a continuous carburizing and annealing facility, it was found that the following problems are involved.
(1) As regards the carburizing rate, it is known from a report by Yo et al. (YO kuun, HARUYAMA shiro et al.: Japan Metallic Society Journal 49 (1985) 7,529) that as shown in FIG. 3, when the amount of C in the metal surface layer portion is large to some extent and the carburizing time is long, since the rate of carburizing is proportional to the rate of diffusion of C into the metal structure after the C concentration reaches an equilibrium concentration between the strip and an atmospheric gas, the rate is normally proportional to a square root of time, and this time carburizing gain area is called as a diffusion-governing area. On the other hand, when the amount of C in the metal surface layer portion is very small and the carburizing time is very short, since the C concentration in the surface layer portion does not reach the equilibrium concentration, the rate of carburizing is proportional to the rate of reaction of the carbon directly on the metal surface layer portion, and this time carburizing gain area is called as a surface reaction-governing area.
Accordingly, for example, when the carburizing conditions for a metal strip are obtained from specifications (Japanese Patent Laid-Open Publication Hei No. 3-199344, etc.) of the metal which is the object of improvement in the anti-secondary working brittleness, since the carburizing concentration and the carburizing depth are very small, in this case it is necessary to perform the carburizing treatment in the surface reaction-governing area, and it was found that the carburizing quantity into the metal strip cannot be controlled by carbon potential (C potential) control by a so-called conventional CO/CO.sub.2, etc., control in which it is considered that the metal strip surface layer portion is always in an equilibrium state with carburizing capability possessed by the atmospheric gas.
(2) Furthermore, generally, the atmospheric gas composition in the carburizing conditions can be obtained by chemical equilibrium. However, in conventional solutions, all the reactions conceivable in a gaseus phase system are listed, and a gas composition is obtained by solving non-linear simultaneous equations from these equilibrium relations of individual reactions. However, it is very difficult to obtain a correct limit of sooting generation from reaction equations in the gaseus phase system.
(3) Furthermore, as to the surface reaction rate mentioned above, there is the report by Yo et al. as described above, however, in this report, the carburizing rate of only CO gas is discussed, and it is impossible to apply to an actual situation of continuous carburizing operation which involves complicated composition.
In this respect, in the continuous annealing and carburizing facility as shown in FIG. 2, since it is necessary to perform a predetermined annealing treatment of the metal strip in the heating zone 2 and/or the uniformly heating zone 3, and to perform a predetermined carburizing treatment in the carburizing zone 4, and to perform a predetermined cooling treatment in each of the cooling zones 5 and 6, it is required to perform temperature (hereinafter described also as plate temperature) control of the metal strip in respective heat treatment zones, for example, by controlling a furnace temperature. In each furnace which constitutes each heat treatment zone, the plate temperature control is performed primarily by heat transfer, however, at the same time, upper and lower limits of the furnace inside temperature (hereinafter described also as furnace temperature) itself are present according to capability calculation of each furnace. For example, in the heating furnace in the heating zone and in the uniformly heating furnace in the uniformly heating zone, upper limit values of the furnace temperature are set from the capability of the furnaces, and an in-furnace time (i.e., it is also heating time or uniformly heating time) of the strip which satisfies the upper and lower limit values is set from heat balance which takes into consideration the heat transfer coefficients among a radiant tube, a furnace wall, a hearth roll, etc., and as a result, a plate-passing speed to satisfy the in-furnace time is set. Also, in the cooling furnace in each cooling zone, a heat transfer coefficient or the like of cooling gas jet is employed as the above-mentioned heat transfer coefficient.
On the other hand, in such a continuous annealing and carburizing facility, various operation conditions are mixed in which, the operation condition is changed at a nonstationary portion, for example, a joint portion of coils, or the like, and thus, in order to satisfy these conditions, it is not seldom to control a plate-passing speed having the most fast response speed. However, no concrete means has not yet been proposed for setting various carburizing conditions in the carburizing furnace with respect to the plate-passing speed which is set from various operating conditions including the plate temperature control in the above-mentioned continuous annealing and carburizing, and it is urgently desired to provide a means for controlling the physical properties and the temperature within the carburizing furnace to achieve the carburizing quantity to meet specification factors required for the steel plate as described above, in particular, under the conditions wherein the plate-passing speed is set.
In order to eliminate the restriction to the plate-passing speed, it may be considered to interpose a louver between respective heat treatment zones. However, it is practically difficult in view of actual problems to install the louver which needs large installation space in the continuous annealing facility which originally requires very large installation space, and in the continuous annealing and carburizing facility which is the continuous annealing facility added with the continuous carburizing facility.
Furthermore, there is a trend that more fine conditions are required as the specification factors of the above-mentioned carburized thin steel plate, and in order to meet such specification factors, it becomes necessary to manage and control the carburizing concentration distribution form of the metal strip surface layer portion, that is, to control even a profile in a depth direction of the carburizing concentration of the surface layer portion. For example, in the steel plate used for vehicles and electrical equipment, in order to perform baking hardening after press work, such characteristics are required in which at the time of press work, the forming property is high by exhibiting the elongation index EI and the deep drawing property r value, and at the time of baking hardening, the strength is improved by exhibiting the baking hardening property BH. At the same time, for these steel plates, the cold-slow-aging index (low AI) which enables to maintain the forming property until the time of performing the press work is required. Accordingly, it is necessary that these steel plates are cold-slow-aging index provided high baking hardening type steel plates (low AI-high BH steel plates) having the deep drawing property. When considering the profile of carburizing concentration in the steel, that is, the distribution state which is required in the case of obtaining the steel plate by the continuous annealing and carburizing of an extremely low carbon steel, it is necessary to increase the carbon concentration in the surface layer to a great extent and to form an optimum C gradient while maintaining the carbon concentration in the inner layer portion in a depth direction of the steel plate to that of the extremely low carbon steel. However, in the control method of the carburizing depth and the distribution form of the carburizing concentration described in the above-mentioned Japanese Patent Publication No. 54-31967, such a carburizing concentration profile is not taken into consideration, and it is impossible to apply this control method itself to the control of the carburizing concentration profile.